The present invention relates to a process of preparing an ion exchange membrane extensively employed in electrochemical industries.
An ion exchange resin is utilized in the form of a particle or a membrane, and the said ion exchange membrane is frequently employed as a diaphragm for industrial electrolysis, including saline solution electrolysis. The ion exchange membrane in recent years has attracted much attention in the application thereof to a fuel cell employing a solid polymer electrolyte membrane as an electrolyte.
Since the ion exchange membrane is generally employed in severe conditions, the chemically resistant membrane having a group of --SO.sub.3 H and the like in the perfluoro-resin has been employed. It is, however, quite difficult to introduce a number of ion exchange groups into the resin while maintaining the softness of the polymer so as not to disadvantageously increase the resistance to ionic conductivity. Since voltage loss increases with an increase in the resistance of the membrane, a decrease in the resistance to ionic conductivity is a significant goal to be received for the development of improved electrolysis and fuel cells.
A transfer mechanism of an ion in the ion exchange membrane may be explained by means of the Following cluster structure model. The ion exchange functional groups having an equivalent weight of about 1000, gather to form a spherical cluster having a diameter of about 4 nm, and the adjacent clusters are connected by a fine path formed by ion exchange groups gathering together. The transfer of an anion and a cation occurs by means of water existing in the path. Even if such a cluster were actually present, only a small number of the ion exchange groups exist on a polymer chain and not all of the ion exchange groups exist in the cluster. If the existing ratio of the ion exchange groups in the cluster can be increased, the ionic conductivity increases to lower the voltage loss in the membrane so that the efficiencies of electrolysis and fuel cells increase.
A composition prepared by dissolving a perfluoro-resin in butanol using an autoclave is available for obtaining Nafion.RTM. type of composition having a sulphonic group as part of the perfluorinated hydrocarbon polymer chain. This composition is employed as a laboratory reagent, an electrolyte of a sensor and material for a fuel cell. However, the resistance to ionic conductivity of a membrane prepared by using this composition by means of a casting method is higher than that of commercially available Nafion.RTM. membranes. This is probably because the cluster structure of the membrane prepared from the composition is inferior to of the commercially available Nafion.RTM. membrane. The performance of the sensor and the fuel cell can be improved if a cluster structure is obtained which is the same as or superior to that of commercially available membranes.